The present invention relates to apparatuses and methods for reproducing information. More particularly, the present invention relates to an apparatus and method for reproducing information from a magneto-optical recording medium by the magnetic domain magnification method.
Magneto-optical recording media have drawn attention as rewritable recording media with large storage capacity and high reliability, and they have started to be put to practical use as computer memories. Currently, standardization of magneto-optical recording media with a storage capacity of 6.1 G bytes is in progress.
The technique of reproducing a signal by magnifying a magnetic domain, which has been transferred from a recording layer to a reproducing layer, by applying an alternating magnetic field during signal reproduction from a magneto-optical recording medium has also been developed (hereinafter, the technique is referred to as the reproduction-by-magnification method). Further, a magneto-optical recording medium capable of recording and/or reproducing a 14 G-byte signal by the technique above has been proposed.
In signal reproduction using the above described reproduction-by-magnification method, a signal is reproduced by transferring a magnetic domain of a recording layer, on which the signal is recorded, to a reproducing layer and magnifying the transferred magnetic domain by an externally applied alternating magnetic field. However, the distribution of double refraction in a substrate of a magneto-optical recording medium and the influence of tilt of the substrate, for example, cause a low frequency xe2x80x9cwavexe2x80x9d or xe2x80x9cwindingxe2x80x9d in a reproduced signal RF as shown in FIG. 28. Although reproduced signal RF can be binarized by comparison with a prescribed threshold value TH, reproduced signal RF cannot be binarized correctly in the presence of such xe2x80x9cwinding.xe2x80x9d
Although high pass filters are effective in removing such low frequency xe2x80x9cwindingxe2x80x9d, they cannot remove xe2x80x9cwindingxe2x80x9d sufficiently.
Japanese Patent Laying-Open No. 3-219427 discloses a binarization circuit capable of obtaining correct binary signals even if the extremes of reproduced signal amplitude are varied. The binarization circuit includes a peak bottom detecting circuit for detecting the maximum and minimum of a reproduced signal, a latch circuit for storing the extremum of the maximum or minimum of the detected amplitude, and a differential comparing circuit for comparing the extremum with the current amplitude, activating its output if the absolute value of the difference is larger than a preset value and the current amplitude is larger than the extremum, inactivating its output if the absolute value of the difference is larger than a preset value and the current amplitude is smaller than the extremum, and otherwise maintaining the state of its output. However, magneto-optical disk recording/reproducing apparatuses using the binarization circuit do not reproduce signals from magneto-optical disks by the magnetic domain magnification method. In signal reproduction, therefore, only a laser beam is directed and an alternating magnetic field is not applied.
An object of the present invention is to provide an information reproducing apparatus capable of obtaining a correct reproduced signal without low frequency xe2x80x9cwinding.xe2x80x9d
According to the present invention, an information reproducing apparatus for reproducing information from a magneto-optical recording medium includes a magnetic head, an optical head, and a reproduced signal detecting circuit. A magnetic head is provided to alternately apply, to a magneto-optical recording medium, a magnetic field in a first direction and a magnetic field in a second direction opposite to the first direction. The optical head is provided to irradiate the magneto-optical recording medium with a laser beam and generate a detected signal according to the reflected beam from the magneto-optical recording medium. The reproduced signal detecting circuit is provided to detect, as a reproduced signal, a difference between the detected signal from the optical head when the magnetic field in the first direction is applied and the detected signal from the optical head when the magnetic field in the second direction is applied.
Preferably, the reproduced signal detecting circuit includes a delay circuit and a differential circuit. The delay circuit is provided to delay the detected signal from the optical head. The differential circuit is provided to detect the difference based on the detected signal from the optical head and the delayed detected signal from the delay circuit.
In the information reproducing apparatus, when the magnetic field in the first direction is applied from the magnetic head to the magneto-optical recording medium, a magnetic domain transferred from a recording layer to a reproducing layer is magnified, and a detected signal is output from the optical head according to the reflected beam from the magnified magnetic domain. When the magnetic field in the second direction is applied from the magnetic head to the magneto-optical recording medium, however, the magnified magnetic domain is erased, and a detected signal is output from the optical head according to the reflected beam from the erased magnetic domain. Since the difference between the detected signal that is generated when the magnetic field is applied to magnify the magnetic domain and the detected signal that is generated when the magnetic field is applied to erase the magnetic domain is detected as a reproduced signal, low frequency xe2x80x9cwindingxe2x80x9d included in the detected signal is offset. As a result, the information reproducing apparatus can obtain a correct reproduced signal.